Light energy, including but by no means limited to laser light energy, has been used in medicine and surgery for many years. Different wavelengths of light interact differently with tissue, so tissue effects are wave-length-dependent. Lasers in particular are used in many different types of medical procedures. Different lasers cause different tissue effects, depending upon the wavelength of the laser emission. Among the types of lasers used in laser medicine are the CO.sub.2 laser, the KTP laser and the neodymium:YAG laser.
Neodymium:YAG (Nd:YAG) lasers have been one of the most popular lasers in laser medicine. The Nd:YAG laser is an efficient and inexpensive source of high-power radiation in comparison to other types of lasers. Thus, cost-effective, high-power laser radiation can be made available at the treatment site. In addition, the absorption coefficient of water, the major constituent of tissue, is near its minimum at the fundamental wavelength (1.06 .mu.m) of the radiation from Nd:YAG lasers. As a result, the radiation from an Nd:YAG laser penetrates deeply into tissue and is excellent for coagulation. Indeed, Nd:YAG lasers were initially used solely for coagulation. However, with suitably high power, laser radiation from Nd:YAG lasers can also be made to vaporize tissue.
In addition to laser light sources, which are referred to as "coherent," non-laser or "incoherent" light sources may also be used for medical and surgical procedures.
In order to get the light energy from the light source to the tissue to be treated, it is desirable to have a delivery system between the light source and the operative site. Such delivery systems as used in medicine and, in particular, in surgery can be broadly divided into those which either contact or do not contact tissue to be treated. In non-contact delivery systems, the distal end of the delivery system does not touch the tissue but, instead, uses a fiber optic or other light guide means to conduct light energy to a location adjacent, but not touching, the tissue. The light energy passes from the guide means through a gas or a liquid before reaching the tissue. The interface between the gas or liquid and the tissue can result in a substantial diffuse reflection of light energy (greater than 40% in some cases) away from the tissue.
To avoid this and other problems with non-contact procedures, techniques and devices have been developed in which the distal end of the delivery system comes into physical contact with the tissue. Direct physical contact between the distal end of the delivery system and the tissue substantially reduces energy losses due to reflection (typically to less than 5%). The reduction in diffuse reflection results in safer surgical procedures. The reduction in diffuse reflection results in less damage to adjacent tissues, and potentially less energy reflected into the surgeon's eyes. Contact procedures, by eliminating reflections, permit a more efficient use of light energy in surgery. Since energy loss is reduced, less power is required and, therefore, smaller, less expensive light sources or lasers can be used.
Contact delivery systems are disclosed in U.S. Pat. Nos. 4,592,353 and 4,693,244, both assigned to the assignee of the present invention.
U.S. Pat. No. 4,592,353 discloses a medical laser probe which has a contact member of laser transmitting material in front of a forward end of a fiber optic laser light guide so as to enable the probe to be used in contact with the tissue. There is a small gap between the forward end of the laser light guide and the rearward end of the contact member.
In the past, if a surgeon desired to use a large contact member for a particular procedure, it was necessary to use either a separate fiber optic and contact member, or a single large-diameter fiber optic with a shaped contact portion. In the first case, Fresnel loss results in a requirement for cooling the junction between the fiber optic and the contact member, and the resulting inefficiencies from the Fresnel loss led to undesirable power loss. In addition, the junction between the fiber optic and the contact member had to be kept well out of the surgical field to avoid burning adjacent tissues. In the second case, where a single fiber was used and the surgeon required a large contact area, the diameter of the contact portion was limited to the size of available fiber optics. This meant that the size of the contact portion was limited, or costly, non-standard, large diameter fibers running the entire distance between the laser source and the contact portion had to be used. In addition, such large diameter fibers are relatively inflexible.
U.S. Pat. No. 4,693,244 discloses another medical and surgical laser contact probe in which the portion of the probe that contacts tissue to be treated is tapered so as to emit laser radiation from the tip end face of the tapered portion substantially without leaking it out from the tapered portion. In one embodiment of the invention disclosed in U.S. Pat. No. 4,693,244, an artificial sapphire contact member is located in front of the forward end of a fiber optic, with a small gap between the forward end of the fiber optic and the contact member. (In another embodiment of the invention of the '244 patent, a single fiber optic is used to both convey laser energy from a laser source and contact tissue to be treated.)
When using two-piece contact delivery systems such as those described above, energy losses occur as the laser energy travels from the fiber optic through the gap on its way into the contact member. Losses occur from energy being reflected back toward the fiber. Such losses range from approximately 0.5 to 12% of the transmitted energy. The magnitude of such losses results in the need for a cooling medium to eliminate unwanted heating which occurs at the gap, where the reflected light energy is converted to heat energy. (These losses do not occur in the single-fiber embodiment of the invention of the '244 patent, since there is no gap.)
Hence, despite the advantages of prior devices for contact procedures, the prior two-piece contact devices still exhibit several shortcomings. Inefficiencies due to losses at the junction between the fiber optic and the contact member require higher power laser sources. Heat at the gap between the fiber optic and the contact member can result in temperatures sufficiently high to burn tissue and damage expensive surgical devices such as endoscopes in which the structure is placed, and can melt the mechanism holding the fiber and the contact member together and result in contact members separating from the device within a patient.
U.S. Pat. No. 4,592,353 recognizes this problem, and discloses cooling the laser probe with a cooling fluid, such as a liquid or a gas. This problem is also recognized by U.S. Pat. No. 4,832,024, which discloses a cooling system in the context of a cardiovascular catheter. In U.S. Pat. No. 4,832,024, coolant is recirculated and does not flow into the surgical field. The coolant in U.S. Pat. No. 4,592,353 is not recirculated, but instead flows into the surgical field. Thus, current methods of eliminating unwanted heat generated by losses at the junction between the optical fiber and the contact member involve the controlled use of coolant fluids which are caused to flow over the area in which the heat is generated. The fluid media are typically gases such as purified nitrogen or air, or liquids such as saline. The coolant fluid is then either allowed to escape out into the surgical field, or is recirculated and either recycled (returned to be re-used for cooling) or allowed to escape, but away from the surgical field. Existing cooling systems require pumps or other means for handling the cooling fluids. Such cooling systems add unnecessary cost in terms of materials and nursing labor to already-costly surgery. They also add to the required training of staff personnel and to the inventory of materials used for surgery. In addition, they are very inconvenient. Moreover, the choice of an inappropriate cooling medium can lead to catastrophic circumstances. If gas cooling is mistakenly utilized in a blood vessel or within a gas-sensitive organ, or if the wrong gas is utilized, severe patient injury or death can result. Even under the best of circumstances, the correct coolant can still cause problems, such as inadvertent cooling of the working region of the contact member by the fluid in the surgical field. Furthermore, coolant creates steep temperature gradients which may induce thermal shock.
Perhaps the greatest disadvantage of prior two-piece delivery system designs is that the heat loss at the gap between the fiber and the contact member is not useful for surgery. It is wasted energy from a costly energy source. It is known that increasing the temperature of the contact member results in elevated tissue temperatures, temperatures greater than would be created by the laser energy alone, which help vaporize the tissue. For example, U.S. Pat. No. 4,736,743, also assigned to the assignee of the present invention, discloses a medical laser probe in which the contact member, which contacts tissue is coated with a material which absorbs a portion of the laser radiation and converts it to heat. The combination of high contact member temperatures and laser radiation makes such coated devices highly effective for vaporizing tissue.
Prior laser probes have also been proposed in which the output end of a fiber optic is either embedded in, or spaced a short distance from, a transmissive contact member. For example, PCT publications PCT/JP90/01122 and PCT/JP90/01079 show such probes. However, neither of those publications recognizes or deals with the issue of heat generated at the interface between the fiber optic and the contact member, and neither of those publications makes any suggestion that such heat can be put to practical use. Indeed, those publications teach away from the concept of using the heat generated at the interface by keeping the interface well away from the working region of the tip, so that the heat can be dissipated by the tip material before it can be used at the working region.
The present invention is based in part on the realization that heat generated at the interface between the fiber optic and the contact member can be used to enhance the therapeutic effect of a contact member by putting otherwise wasted energy to use in raising the temperature of the contact member.